Traditionally, light-emitting diodes have been used mostly as indicator lights for a wide variety of devices. Due to advances in diode light intensity and color quality, modern LEDs are being used increasingly for area lighting applications, such as in indoor or outdoor lighting fixtures.
Reflectors for LED and other lighting systems are typically metal or at least metal coated. Metal reflectors for lighting applications, most commonly incorporating aluminum and more rarely silver or other metals on the reflective surface, are widely used because their reflectivity is largely insensitive to the incident angle of the light and they are amenable to the construction of a range of reflector shapes. Metals, though, exhibit limited reflectivity due to surface finish, degradation during processing (e.g. oxidation and surface contamination), and the application of protective coatings.
Other reflectors include a coating of thin films, such as polymeric or dielectric layers. The latter type can be referred to as a dielectric mirrors, or Bragg reflectors. A related term is a dichroic stack, mirror, or reflector.
Polymer stacks have shortcomings including relatively low robustness. Polymer stacks, being an example of non-dielectric stacks, can operate decently within only limited temperature range, for example. The polymer stacks can also break down when exposed to ultraviolet (UV) light, which can be generated by an LED or a high-intensity discharge (HID) light source. Non-dielectric stacks, e.g., polymer stacks, may also susceptible undesirably to water uptake and swelling.
Conventional dielectric optical stack reflectors are designed for reflecting optimally only light impinging on the reflector within a limited range of incident angles. An example range is centered about forty-five degrees (45°), such as a range of 40° to 50°. Especially with multi-component light sources, such as LED arrays or clusters within a single lighting apparatus, though, light rays can arrive at intra-apparatus reflectors at a wide variety of incident angles.
While conventional lighting systems are designed so that some reflectors, or portions of a reflector, receive light arriving within the optimal range, other reflectors or portions receive light at angles outside of the optimal range. Incident angles outside of the optimal angles can approach 0° on the low end and 90° on the other. Rays arriving outside of the optimal range reflect with an undesirable shift in color, and less-than-desirable intensity.
Losses in light quality (color and intensity) can be quantified. Turning to the figures and, more particularly to the first figure, FIG. 1 is a data chart 100 showing reflectivity, or reflectance (y-axis 102), or percentage of light reflected, of various wavelengths (x-axis 104) of two rays of light being the same, and reflecting off of the same surface, but at two different angles. A first 106 of the two rays impinges on the reflecting surface at 60° and a second ray 108 impinges at only 15°.
As can be seen in FIG. 1, a relatively high percentage of the light between about 380 nanometer (nm) and 625 nm wavelengths is reflected for both rays 106, 108 (i.e., at both angles). Wavelengths at the lower end of the visible spectrum, e.g., from slightly above 400 nm to below, include indigo and violet, while orange starts at the other end from yellow around 600 nm, transitioning to more red approaching and exceeding 650 nm.
Within this context, FIG. 1 depicts that while the reflectivity, or reflectance 102 (y-axis value) for the first ray 106 (the 60° incident ray) remains high for even the higher, more red, wavelengths of the ray 106, the reflectance for the second ray 108 (the 15° incident ray) plummets for the higher wavelengths of the ray 108. The result is undesirable color, including insufficient red light, and, so, less overall intensity and color uniformity for the light 108 being reflected after arriving at the lower angle.